BOLT GRADE MARKING AND STRENGTH CHART

UNIFIED BOLTS
  MECHANICAL PROPERTIES
HEAD MARKING GRADE AND MATERIAL Nominal Size Range
(inches)
Proof Load
(psi)
Min. Yield Strength
(psi)
Min. Tensile Strength
SAE Grade 2 Bolt Markings
NO MARKINGS

GRADE 2

Low or medium carbon steel

1/4 thru 3/4 55,000 57,000

74,000 psi

510 Mpa

Over 3/4 thru 1-1/2 33,000 36,000

60,000 psi

414 Mpa

SAE Grade 5 Bolt Markings
3 RADIAL LINES

GRADE 5

Medium Carbon Steel,
Quenched and Tempered

1/4 thru 1 85,000 92,000

120,000 psi

828 Mpa

Over 1 thru 1-1/2 74,000 81,000

105,000 psi

724 Mpa

SAE Grade 5.2 Bolt Markings
3 RADIAL LINES

GRADE 5.2

Medium Carbon Alloy Steel,
Quenched and Tempered

1/4 thru 1-1/2 120,000 130,000

120,000 psi

828 Mpa

SAE Grade 7 Bolt Markings
5 RADIAL LINES

GRADE 7

Medium Carbon Alloy Steel

1/4 thru 1-1/2 120,000 130,000

133,000 psi

917 Mpa

SAE Grade 8 Bolt Markings
6 RADIAL LINES

GRADE 8

Medium Carbon Alloy Steel,
Quenched and Tempered

1/4 thru 1-1/2 120,000 130,000

150,000 psi

1034 Mpa

SAE Grade 8.2 Bolt Markings
6 RADIAL LINES

GRADE 8.2

Medium Carbon Alloy Steel,
Quenched and Tempered

1/4 thru 1-1/2 120,000 130,000

150,000 psi

1034 Mpa

ISO property classes for steel bolts, screws, studs and nuts

The strength of standard ISO metric steel bolts, studs and nuts is readily identified by means of a numerical code.

Property classes for steel bolts, screws and studs

The code is comprised of two numbers separated by a dot. This dot is not a decimal marker but is merely a means of separating the two parts of the code. The number to the left of the dot when multiplied by 100 provides an indication of the Ultimate Strength in Mega Pascals (MPa) while the number to the right when multiplied by 10 times the preceeding number gives the Yield Strength (Mega Pascals - MPa) or Stress at 0,2% Permanent Set (MPa) depending on the property class; e.g. 8,8

Ultimate Strength 8 x100 = 800MPa

Yield Strength or Stress at 0,2% permanent Set 8 x 8 x 10 = 640 MPa

Property class Designation for Nuts

The corresponding code for nuts is a single number and is derived from the left hand number of the property class of the bolt, screw and stud. This number when multiplied by 100 indicates the Proof Load Stress of the nut in MPa. Thus, for the bolt of property class 8,8 in the above example a nut of property class 8 is required. This nut will have a Proof Load Stress of 8 x 100 = 800 MPa

In practice only standard threaded fasteners in the high tensile property classes, i.e. property class 8,8 and above are required to be marked with the appropriate code.

METRIC BOLTS
  MECHANICAL PROPERTIES
HEAD MARKING GRADE AND MATERIAL Nominal Size Range (mm)     Min. Tensile Strength
Class 4.6 Bolt Markings
NO MARKINGS

CLASS 4.6

Low or Medium Carbon Steel

5mm - 68mm 225 235

400 MPa

58015 psi

Class 4.8 Bolt Markings
NO MARKINGS

CLASS 4.8

Low or Medium Carbon Steel

1.6mm - 68mm 310 315

430 Mpa

62366 psi

Class 5.8 Bolt Markings
NO MARKINGS

CLASS 5.8

Low or Medium Carbon Steel

5mm - 68mm 380 392

520 Mpa

75419 psi


8.8

CLASS 8.8

Medium Carbon Steel, Quenched and Tempered

All Sizes below 16mm 580 628

800 Mpa

116030 psi

16mm - 72mm 600 628

830 Mpa>

120381 psi

Class 9.8 Bolt Markings
9.8

CLASS 9.8

Medium Carbon Steel, Quenched and Tempered

1.6mm - 68mm 650 660

900 Mpa

130533 psi

Class 10.9 Bolt Markings
10.9

CLASS 10.9

Alloy Steel, Quenched and Tempered

5mm - 68mm 830 833

1040 Mpa

150839 psi

Class 12.9 Bolt Markings
12.9

CLASS 12.9

Alloy Steel, Quenched and Tempered

1.6mm - 68mm 970 1060

1220 MPa

176946 psi

Class 14.9 Bolt Markings

14.9

CLASS 14.9

Alloy Steel, Quenched and Tempered

1.6mm - 68mm 1090 1236

1400 MPa

203052 psi

STAINLESS BOLTS
  MECHANICAL PROPERTIES
HEAD MARKINGS CLASS AND MATERIAL Nominal Size Range(mm) Proof Load (MPa) Min. Yield Strength (MPa) Min. Tensile Strength (MPa)

A2 Grade Bolt Markings

A2

A2 Stainless

Steel alloy with 17- 19%
Chromium and 8-13% Nickel

Class 50 1.6 - M39 500 210 Min.

500 Mpa

72518 psi

Class 70 1.6 - M24 700 450 Min.

700 Mpa

101526 psi

Class 80 1.6 - M24 800 600 Min.

800 Mpa

116030 psi

A4 Grade Bolt Markings

A4

A4 Stainless

Steel alloy with 17- 19%
Chromium, 8-13% Nickel
and molybdenum 3%

Class 50 1.6 - M39 500 210 Min.

500 Mpa

72518 psi

Class 70 1.6 - M24 700 450 Min.

700 Mpa

101526 psi

Class 80 1.6 - M24 800 600 Min.

800 Mpa

116030 psi

Tensile Strength: The maximum load in tension (pulling apart) which a material can withstand before breaking or fracturing.

Yield Strength: The load at which a material exhibits a specific permanent deformation.

Proof Load: An axial tensile load which the product must withstand without evidence of any permanent set.